Well, since there are potentially 17 million in this situation (if they were all similar), now we're talking over $14 billion. That might be enough to get their attention.
What is needed is a lot of publicity, and maybe someone to publish the details on how the case was won, so others could easily follow without investing a lot of time in research; a little 'collect your $$$ from AT&T kit'.
I love their excuse too... taxing their network too much, huh? Well, if that is really the case for people using 2 GB, then their network needs some serious work. They need to stop selling accounts until they get this resolved. If they don't do that, then this is simply a ploy to push people off of the unlimited plans. It's probably both.